The present invention relates to a cold isopressing method and mold for compacting a granular ceramic material for use in manufacturing ceramic tubes. More particularly, the present invention relates to such a method and mold in which the ceramic tubes are useful in forming ceramic membrane elements of the type exhibiting infinite hydrogen or oxygen selectivity. In other aspects, the present invention relates to a cold isopressing method and mold in which ceramic tubes are formed having enlarged end portions that are more amenable to sealing than prior art ceramic tubes.
Ceramic tubes have many industrial uses, for instance, insulators, filters, etc. An important recent use involves the employment of such tubes in membrane modules to separate oxygen or hydrogen from a gas stream. The ceramic tubes used in membrane separation applications are manufactured from materials that exhibit infinite oxygen or hydrogen selectivity at high temperatures. In an oxygen-selective membrane, oxygen is ionized at one surface of the membrane to form oxygen ions. The oxygen ions travel through the membrane to the opposite surface thereof where the oxygen ions recombine to form elemental oxygen. In forming the elemental oxygen, electrons are given up from the ions. Depending upon the type of material used in fabricating the membrane, the electrons either flow through the membrane material itself or through separate conductive pathways to initially ionize the oxygen.
The traditional method fabricating ceramic tubes involves processes such as slip casting or extrusion. The use of extrusion to form solid-state membrane modules is disclosed in U.S. Pat. No. 5,599,383.
Another known method of fabricating ceramic tubes is isopressing. In cold isopressing, a tubular mold is utilized that is formed from a soft neoprene rubber. This mold, known as a bag, is used in conjunction with a mandrel that projects into the bag. The bag is filled with a ceramic material in granular form. The mold is subjected to a hydrostatic pressure within a vessel containing cold or warm water that is sufficient to compact the ceramic material into a green ceramic tube. After compaction, the hydrostatic pressure is relaxed and the green ceramic form is removed from the mold. In this regard, the mandrel is provided with a slight taper to permit separation of the green ceramic tube from the mandrel. The green ceramic tube can then be heated to burn out organic binder materials and the like and to sinter the ceramic.
The prior art method of isopressing has found application in the manufacture of short thick tubes. When longer tubes are attempted by this method, defects are found in the fired and sintered tubes. The reason for such defects is that it is impossible to introduce ceramic powder into the mold so that the powder is uniformly distributed. For instance, if there exists a slight wrinkle in the mold, the powder will tend to hang up on the wrinkle to produce a defect in the finished ceramic tube.
As will be discussed, the present invention in one aspect provides a cold isopressing method and mold that is particularly applicable to forming long thin, ceramic tubes that can be used in ceramic membrane applications. As will also be discussed, other aspects of the present invention are particularly useful in the fabrication of ceramic tubes having end configurations that are more amenable to sealing than prior art sealing methods.
The present invention provides a cold isopressing method for compacting a granular ceramic material. As used herein and in the claims, the term xe2x80x9cgranular ceramic materialxe2x80x9d means ceramic powder or a mixture comprising a ceramic powder, an organic binder and a plasticizing agent. In accordance with such method, the granular ceramic material is introduced into an isopressing mold having a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. After the cylindrical pressure bearing element is sealed, the cylindrical pressure bearing element is subjected to a hydrostatic pressure to compact the granular ceramic material. The hydrostatic pressure is relaxed after such compaction. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape during the introducing of the granular ceramic material and is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon the application of the hydrostatic pressure and to at least substantially return to its original shape upon the relaxation of the hydrostatic pressure.
The resiliency of the cylindrical pressure bearing element allows retraction of the cylindrical pressure bearing element from the granular ceramic material after compaction. In this regard, the use of the term, xe2x80x9cat least substantiallyxe2x80x9d with respect to the return of the cylindrical pressure bearing element to its original shape admits to the possibility that after relaxation of hydrostatic pressure, the cylindrical pressure bearing element might be slightly out of round. However, such cylindrical pressure bearing element should not be so out of round that the mold is unable to completely retract from the granular ceramic material.
As may be appreciated, the provision of a cylindrical pressure bearing element that will maintain its shape during filling allows for long thin tubes to be fabricated with the ceramic material having a uniform density along the length of the tube. Gross maldistributions of ceramic material that are caused by the wrinkling of the cylindrical pressure bearing element during filling are eliminated. Moreover, the resiliency of the cylindrical pressure bearing element ensures a clean separation of the mold from the compacted ceramic material to also prevent tube defects.
The at least one mandrel can be a single mandrel to produce a tube-like structure. Alternatively, multiple, parallel mandrels can be used to form a cylindrical structure having internal passageways. Both types of structures, the tube or the cylindrical structure having internal passageways would be useful in forming ceramic membrane elements.
The at least one mandrel can be connected to an enlarged base element that projects into one end of the cylindrical pressure bearing element, thereby to seal the cylindrical pressure bearing element at the one end. The granular ceramic material is introduced into the isopressing mold through the other end of the cylindrical pressure bearing element during filling. A removable end plug is positioned within the other end of the cylindrical pressure bearing element to seal such other end thereof. In this regard, the term xe2x80x9cremovablexe2x80x9d as used herein and in the claims means that such end plug can be removed and is not permanently bonded attached to the cylindrical pressure bearing element.
One end of the cylindrical pressure bearing element can be provided with an enlarged, axial end bore positioned so that the compacted granular ceramic material has an enlarged end section. As will be discussed, such an enlarged end section can be used in a sealing arrangement at the connection of a finished ceramic tube to a tubesheet.
Preferably, the isopressing mold is vibrated while the granular ceramic material is introduced into the isopressing mold. The vibrations act on the cylindrical pressure bearing element member to help fill the mold and ensure that there is no hang up of the granular ceramic material within the mold. Such transmission of vibrations is possible due to the rigidity of a cylindrical pressure bearing element in accordance with the present invention as opposed to a soft rubber bag of the prior art. As may be appreciated, the use of such a vibration technique during filling is particularly important in the fabrication of long, thin tubes.
In case of a single mandrel, the cylindrical pressure bearing element can advantageously be sized such that the granular ceramic material prior to compaction occupies an annular space having a wall thickness no less than about twice that of the granular ceramic material after compaction.
In granular ceramic materials containing a binder, the binder can comprise no more than about 5% by weight of the granular ceramic material. This is to be contrasted with ceramic tubes of the prior art in which binder is 10% or more by weight of the granular ceramic material. The greater the binder content, the greater the time expended in firing and sintering a green ceramic tube formed of compacted, granular ceramic material and also, the greater possibility of tube defects.
As stated above, the present invention is particularly applicable to forming membrane units for separating oxygen and hydrogen. As such, the granular ceramic material can be the type that is a capable of conducting one of oxygen ions and hydrogen ions. These membrane elements are preferably in the form of long thin tubes. However, the formation of long thin cylindrical elements in accordance with the present invention are possible. In order such manufacture tubes, a cylindrical pressure bearing element can be provided with a length of no less than about 60 millimeters and a diameter such that the granular ceramic material prior to compaction occupies an annular space having a radial thickness of no more than about 4 millimeters. Unlike the prior art, the single mandrel used in a mold to form a tube need not be tapered and can be of cylindrical configuration.
In another aspect, the present invention provides an isopressing mold for compacting a granular ceramic material. The isopressing mold can be provided with a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. If tubes are to be formed, the at least one mandrel can be a single mandrel. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape upon introduction of the granular ceramic material into the cylindrical pressure bearing element and is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon an application of a hydrostatic pressure and to at least substantially return to its original shape upon the relaxation of the hydrostatic pressure. As stated above, this allows the cylindrical pressure bearing element to retract from the granular ceramic material after compaction thereof by the hydrostatic pressure. A means is provided for sealing the cylindrical pressure bearing element at opposite ends thereof.
Preferably, the sealing means comprise an enlarged base element connected to the at least one mandrel and positioned within one end of the cylindrical pressure bearing element, thereby to seal the cylindrical pressure bearing element at the one end thereof. A removable end plug is positioned within said other end of said cylindrical pressure bearing element to seal such other end.
One end of the cylindrical pressure bearing element can be provided with an enlarged, axial end bore positioned so that the granular ceramic material after having been compacted has an enlarged end section. In case of a single mandrel, the cylindrical pressure bearing element can be sized such that the granular ceramic material prior to compaction occupies an annular space having a wall thickness no less than about twice that of the granular ceramic material after compaction.
The isopressing mold can further comprise a rigid support attached to the cylindrical pressure bearing element to ensure that the cylindrical pressure bearing element remains straight during mold filling with the granular ceramic material. Preferably, the material making up the cylindrical pressure bearing element is polyurethane having a hardness of 95A on the durometer scale.
As stated above with respect to a single mandrel embodiment of the present invention, the cylindrical pressure bearing element can advantageously have a length of no less than about 60 millimeters and a diameter such that the granular ceramic material prior to compaction occupies an annular space having a radial thickness of no more than about 4 millimeters.
Unlike isopressing molds of the prior art in which the mandrel is tapered, an embodiment of the present invention employing a single mandrel can employ such mandrel with a cylindrical configuration throughout.
As mentioned previously, short ceramic elements can be readily manufactured by the use of prior art isopressing techniques. Such prior art techniques can be advantageously adapted in accordance with the present invention to produce ceramic tubes having enlarged end sections for sealing purposes. In this regard, further aspects of the present invention involve the manufacture of green ceramic tubular forms having enlarged end sections for sealing purposes.
In one of these further aspect, a cold isopressing method is provided for compacting a granular ceramic material into a green ceramic tubular form having an enlarged end section. In accordance with such method, the granular ceramic material is introduced into an isopressing mold having a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. If tubes are to be formed then the at least one mandrel can be a single mandrel. The cylindrical pressure bearing element is sealed and subjected to a hydrostatic pressure to compact the granular ceramic material into the green ceramic tubular form. Thereafter, the hydrostatic pressure is released. The cylindrical pressure bearing element has an enlarged end bore to produce the enlarged end section of the green ceramic tubular form.
In another of the further aspects of the present invention, an isopressing mold is provided for compacting a granular ceramic material into a green ceramic tubular form having an enlarged end section. In accordance with such aspect, the isopressing mold comprises a cylindrical pressure bearing element and at least one mandrel (a single mandrel if tubes are to be formed) located within the cylindrical pressure bearing element. The cylindrical pressure bearing element has an enlarged end bore to produce the enlarged end section within the green ceramic tubular form. A means is provided for sealing the cylindrical pressure bearing element at opposite ends thereof.